1. Field of the Invention
This invention relates to improvements in a shaft seal arrangement for a turbocharger, and more particularly to a shaft seal arrangement for preventing oil leak from a bearing side to a compressor side of the turbocharger.
2. Description of the Prior Art
A turbocharger installed in an internal combustion engine is usually provided with a shaft seal arrangement for preventing lubricating oil supplied to bearings from leaking out to the side of a compressor of the turbocharger. The bearings support a rotatable shaft on which a turbine and compressor wheels are fixedly mounted. Such a shaft seal arrangement is disclosed, for example, in Japanese Utility Model Provisional Publication Nos. 60-116035 and 61-166134, in which the rotatable shaft of the turbocharger is provided with two seal rings for preventing oil leakage. An air supply passage is formed to supply air into between the two seal rings. In this case, oil leakage from the bearings is suppressed by cancelling a pressure difference between the seal rings and the bearing housing.
However, when a throttle valve of an engine is disposed upstream of the turbocharger compressor, a greater vacuum is developed in the compressor in an engine operating condition in which the throttle valve is closed. As a result, the amount of air sucked out from between the seal rings to the side of the compressor wheel increases, so that air pressure between the seal rings lowers due to a pressure drop of air flowing through the air supply passage. This causes lubricating oil to leak from the bearing housing to the compressor wheel side of the turbocharger.